I have just re-registered with Avast 4 and this is my first time on this forum so I apologise if this item has been covered before. When I tried to run my first virus check with the newly registered version of Avast 4.7, the scan runs OK to a particular file and then “freezes”. This happens every time I try to run a virus scan, and the freezing occurs at exactly the same file. I had no problems with running Avast 4.7 prior to re-registering. Can anyone help - I am not very technically minded so a “fools guide” would be most helpful. 
Maybe it doesn’t freeze - it just takes a LOONG time to scan a given file? (e.g. a huge file)
Did you leave it run for a long time?
Thanks
Vlk